The present invention generally relates to a probability estimation table apparatus, and more particularly to an apparatus for reading a probability estimation table for use in an arithmetic coder as well as an apparatus for generating the probability estimation table stored in a memory of the probability estimation table reading apparatus.
The ISO/IEC International Standard of Coded Representation of Picture and Audio Information (Early Draft, WG9-S1R2, Dec. 14, 1990, the standard number being still not assigned) defines a method for compression encoding a bi-level image, and recommends a standard probability estimation table adaptive to the arithmetic coding data compression. The standardization of this technology is under way, and some prototypes of arithmetic coders for carrying out the arithmetic coding data compression have been proposed. However, there is still not a useful mechanism for outputting a next index, required for compression encoding an image, by using the standard probability estimation table.
FIGS. 1A and 1B show the standard probability estimation table recommended by the above mentioned international standard. The probability estimation table is called the Qe table. For each possible value of the index CX there is stored a 1-bit value MPS (CX) and a seven-bit value ST (CX), which together completely express the adaptive probability estimate associated with that particular index. An index CX is used to identify the index of the state of the coder for coding a current pixel.
In the standard Qe table in FIGS. 1A and 1B, four arrays are indexed by the index (CX). The MPS is the estimated most likely color for a pixel PIX in a symbol sequence. The Qe Value is a 15-bit value of the LPS interval size, which can be interpreted to be a probability based on a prescribed equation.
The arrays N/I-LPS (Next Index-LPS) and N/I-MPS (Next Index-MPS) are 7-bit values of next indexes giving, respectively, the next Qe state for an observation of the LPS and the MPS. The movement given by the N/I-MPS only occurs if in addition to observing the MPS, a renormalization also occurs. When the movement given by the N/I-LPS occurs, there will also be an inversion of MPS (CX) if the SWITCH (CX) value is 1. The array SWITCH is a 1-bit value of the MPS/LPS switch. A Qe table reading apparatus requires a memory (for example, a ROM) for storing the above mentioned standard Qe table. Since the 1-bit data of the switch and the 15-bit data of the Qe-Value are fixed and cannot be changed, a memory space of at least 112 14-bit words is required for storing the LPS data and the MPS data of the standard Qe table. However, memory chips currently available to the manufacturers at a low cost are multi-purpose 8-bit ROMs. There is a problem in that it is necessary to use a 16-bit memory chip for storing the standard Qe table with space of at least 112 14-bit words, thus increasing the manufacturing cost.